The invention relates to electronic ballasts for operating discharge lamps such as fluorescent lamps at a high frequency, and in particular to such ballasts having a minimum number of active components.
Most lamp ballast inverters are manufactured in large quantities for sale in a highly competitive market, so that reliability and cost are primary considerations. Half-bridge inverters are widely used because they have a relatively low parts count and high efficiency. A particularly effective type of electronic ballast, or converter, has a load circuit using a resonant inductor or transformer having a linear core, generally together with MOSFET switches (metal oxide silicon field effect transistors). In this context a linear core is one in which under all normal operating conditions a significant increase in magnetizing current will be accompanied by a significant increase in flux level. However, because of the switching action of the diodes and inverter transistors, the circuit operation is only piecewise linear during different stages of high frequency and line voltage cycles.
Many circuit modifications have been proposed to improve line current power factor while keeping lamp current crest factor within acceptable limits. For example, it is known to vary inverter frequency during each cycle of the low frequency input power. Most early proposals for improved performance of electronic ballasts have involved substantial additional circuitry, but in the last ten years a number of relatively simple high frequency power feedback circuits have been developed to cause the rectifier diodes for the DC bus supplying the inverter to conduct over substantially the entire low frequency cycle. In general, these feedback circuits either couple some or all of the load current to one of the inverter terminals, or couple a high frequency voltage from the inverter or load circuit through a feedback capacitor to one of these terminals.
However, with known power feedback circuits, the lamp ballast designer has been forced to make undesirable trade-offs between lamp crest factor, line current power factor, and circuit cost and complexity. A further complicating factor is the desirability to save power by dimming the lamps when less lamp brightness is needed.
Examples of power feedback are shown in U.S. Pat. No. 5,608,295, where a voltage, equal to the voltage across a resonant capacitor C8 plus a portion of the lamp voltage across a matching transformer, is fed through a capacitor 2A to one input terminal of a voltage doubler power supply. The tap 1T is at a location along the winding such that the voltage has a greater amplitude than the input line voltage so that during a part of each high frequency cycle one or the other of the rectifier diodes conducts. FIG. 1 shows a full bridge rectifier embodiment, with similar feedback to a node between two capacitors C2A and C2B in series across the line input to the bridge.
Feedback of this type has the disadvantage that, if inverter frequency is raised in order to dim the lamp, or rises as a result of removal of the lamp (or removal of a lamp in a multiple lamp arrangement), the feedback increases and tends to increase the DC bus voltage. This increases the stress on all the components, and reduces reliability or increases cost because the components have higher ratings than would otherwise be required.
An object of the invention is to provide a low frequency to high frequency converter for driving a variable load, which avoids DC bus over-boosting at light loads.
Another object of the invention is to provide such a converter for use as a fluorescent lamp ballast.
Yet another object of the invention is to provide a fluorescent lamp ballast which avoids overboosting if frequency is raised for dimming purposes.
According to the invention, a high frequency power converter includes a DC supply circuit which receives low frequency power, through an input network, from a source of low frequency voltage. A bulk storage capacitor circuit maintains the DC voltage from the supply circuit substantially constant during a cycle of the low frequency line voltage. A high frequency voltage source is connected to receive power from that DC voltage. A feedback network is connected between the high frequency voltage source and a node at the low frequency power side of the DC supply circuit. This network forms part of a feedback path which has an inductive impedance at one or more frequencies within the operational frequency range of the high frequency source.
A power converter according to this general description has the advantage that, at higher than normal operating frequencies within a range of the voltage source operation, the total impedance in the feedback path increases. This characteristic may reduce excessive DC bus voltage during operation with no load or reduced load. Additionally, with respect to harmonics of the high frequency of the voltage source, inductive feedback causes the feedback current to be more sinusoidal than with capacitive feedback. As a result, the input capacitor across the low frequency power source to the rectifier may be smaller.
In a first preferred embodiment, the high frequency voltage source is a connection to a load circuit supplied from the output of a half-bridge inverter. Still more preferably, the load circuit includes a resonant inductor and connection points for a load, the feedback network being connected to receive a voltage proportional to the load voltage.
In a fluorescent lamp ballast according to this first embodiment of the invention, the fluorescent lamp is connected to the load connection points, directly or through a matching transformer. The matching transformer may be a step-up transformer having a high output voltage. A resonant capacitor is connected in parallel with the lamp, and/or a small capacitor may be connected in series with the lamp. The use of the step-up transformer enables operation of more than one lamp without need for a special selective starting circuit, so long as each lamp has its own series capacitor.
In lamp ballasts according to the invention, line current waveform is less impacted by frequency modulating to improve crest factor, than if feedback is purely capacitive. In a further preferred embodiment of a lamp ballast, the feedback network includes a capacitor in series with the parallel combination of an inductor and a capacitor. Thus in this embodiment the inductive impedance in the feedback path is located in the feedback network. Preferably, the input network is a low pass filter having at least one capacitor connected to an AC input terminal of the DC supply circuit. The DC supply circuit is a bridge rectifier, and the network is connected between a load connection point and the AC-input node between two of the diodes. This embodiment has the particular advantage that current through the diodes can be balanced.
In a variation of this embodiment, a similar feedback network is connected to a node between two capacitors which are in series across the low frequency input to the rectifier circuit.
In a second preferred embodiment, the input network comprises two inductive elements magnetically coupled in series, one end of one of the inductive elements being connected to an input terminal of the rectifier. The feedback network is formed by a capacitor connected between the load circuit and the junction or node between the inductive elements. Thus in this embodiment inductive impedance in the feedback path is located in the input network. A lamp ballast with a resonant load circuit as described above, according to this embodiment, has the additional advantages that the peak currents through the rectifier diodes can be reduced, and there is more direct energy transfer through the feedback inductor to the load so that ballast efficiency is improved.
In a third embodiment, the feedback network is connected between the output of a half-bridge inverter and the node at the low frequency power side of the DC supply circuit. The feedback network may comprise simply an inductor and a capacitor in series.
In each of these embodiments, the inductance in the feedback network is much smaller than the resonant inductor or inductors customarily used in EMI networks, but is sufficiently large that the equivalent value of the impedance in the feedback path rises with frequency in at least a portion of the frequency range of the inverter during at least one operating mode, such as start-up, lamp dimming, or ballast operation with a lamp removed or non-operating. The actual values of inductance will be determined, of course, partly according to the designed load power, the normal operating frequency of the inverter, and the voltage of the low frequency power source.